Known in the art is a relatively recent method of treatment known as photodynamic therapy. During photodynamic therapy (PDT), a photosensitizer is introduced into an organism and is activated by light to induce cytotoxicity as shown in FIG. 1. The activated photosensitizer reacts with oxygen species producing singlet oxygen, which then react with tissue. A simplistic scheme for this reaction may be written as,1PS+hv→3PS*+3O2→1O2*+S→S(O)where 1PS is the ground state photosensitizer, hv is a photon of light, 3PS* is the photosensitizer in the excited triplet state, 1O2* is singlet oxygen and S is the biological substrate. More generally, the reaction dynamics follow Type I or Type II pathways to varying extents, as shown in FIG. 2.
The medicinal effect of a PDT photosensitizer (PS) occurs after the substance absorbs light and enters an excited state (reaction step 1). A portion of this energy is released immediately in the form of fluorescent emissions (reaction step 2), while other phosphorescent emissions (reaction step 4) occur after the excited PS converts from a singlet to a triplet state via inter-system crossing (reaction step 3). Typically, the PS triplet state may interact with ground state oxygen molecules, O2 (3Σu), present in tissue to yield cytotoxic singlet state oxygen, (1Δg), via the Type II pathway (reaction steps 5 and 6). This energetic form of oxygen then rapidly reacts with proteins, lipids and DNA (denoted by S in FIG. 2) yielding tissue damage. Additionally, singlet oxygen may react with the photosensitizer itself in a process known as photobleaching (reaction step 7). The PS triplet state may also react via the Type I pathway with other species that initially form radicals and radical ion pairs (reactions steps 8 through 17). These radicals may react with oxygen leading to the production of other cytotoxic compounds such as superoxides (reaction step 16) and the hydrogen dioxide radical anion (reaction step 17). Since oxygen rapidly quenches the excited triplet state of the PS, the Type II mechanism dominates the overall reaction. The reader should appreciate that the reaction schemes covered by the teachings of FIG. 2 are only exemplary and by no means represent the totality of all pathways and do not represent a sequence of mechanistic steps.
After introducing a photosensitive agent into the body, it may remain in its free state or bind to proteins and DNA. These binding processes have an effect on the excited singlet and triplet state lifetimes of the PS and generally complicate optical schemes for in vivo monitoring during photodynamic therapy. During treatment, the physico- and bio-chemical behaviour of photosensitive agents are also quite variable. The effects of photobleaching, tissue oxygenation along with changes in the local chemical environment and protein-binding characteristics of a PS limit the efficacy of photodynamic treatment. Additionally, some PS compounds prefer to collect in the vascular system, while others preferentially concentrate in the lysosomes and near the mitochondria of certain cancerous cells. Since PS bio-uptake and chemistry affects treatment efficacy, drug formulation, and light dosing must be optimized. Unfortunately, the effectiveness of a photodynamic therapy session is usually not evident until after treatment. Furthermore, researchers involved in developing new PDT compounds often have no real time or direct way of knowing how well a drug performs in vivo. Presently, one determines the effectiveness of the drug by evaluating changes in tumour size, the extent of cell apoptosis and tissue necrosis and immune response with analytical bioassays.
In order to better understand the direct chemical reaction dynamics of a PDT compound, several researchers have started to monitor luminescent emissions from tissue during treatment. By measuring the oxygen concentration electrochemically (with the aid of a minimally invasive oxygen electrode), researchers have found a correlation between the change in phosphorescence lifetime or total luminescence intensity with tissue oxygen concentration. This method however is not ideal and produces biased results. Additionally, only the overall oxygen consumption rate in tissue is measured. Other researchers have tried to measure the luminescent emissions of singlet oxygen at 1269 nm, which may directly measure the reaction rate via the Type II pathway. However this measurement is extremely difficult to perform quantitatively in vivo due to the masking effect of other lumiphores and absorbers present in this wavelength region.
Other approaches for estimating the extent of a photodynamic reaction via the Type II pathway appear possible and are, in part, the subject of the present invention.
In literature pertaining to the effect of extremely low frequency (˜50 Hz) electromagnetic fields on the efficacy of a photodynamic treatment, it is known that field strengths of 6 millitesla (mT) may enhance cell death by 20 to 40%. In literature pertaining to the effect of magnetic fields on chemical kinetics, it is known that radical ion pairs, neutral radicals and the potential energy surfaces of triplet-triplet annihilation reactions may be affected by an external magnetic field. A review of pertinent chemical literature has revealed that triplet-triplet annihilation reactions between planar metallophthalocyanine moieties are mildly affected by strong magnetic fields (˜7 T). The literature also suggests that the production of free uncharged radical pairs in solution is mildly affected by weak to medium strength magnetic fields (<0.5 T). In general, these reactions include those of reaction steps 14 and 17 in FIG. 2. The reaction rates of triplet state radical anion-cation pairs appear however to be strongly affected by weak magnetic fields (˜0.01 T). This reaction is shown as reaction step 9 in FIG. 2. Furthermore it is known that the lifetime of triplet state radical anion-cation pairs can be increased by confining them in a membrane or in a micelle. In the present invention, the use of magnetic fields is employed to affect the rate of triplet-triplet annihilation reactions, and for modifying the spin state populations of charged and uncharged radical pairs.